Which are best? Stock, High or Low compression pistons?

[inspector-111]

O.K., I'm about to order an engine kit (still in the very slow rebuild process), and I need to know if I should stick with stock, use higher or use lower compresion pistons. Is there any advantage to changing this if you are going to run about 15psi? (for now, hopefully more later when I have the means to tune way down the road. . .)
This is my DD and will need to be reliable. I have polished the rods (too poor to buy upgrades) and so far have SS valves with stock springs and cams.

Thanks for any help!

Chris.

 

[gsxeclipse97]

I wanted to run 8.8 but the machine shop was so against the idea of running something higher than stock he wouldn't warranty it. so i just got the 8.5; No matter like its said above that CR is one of the last factors you should worry about even though you would have more tq in the low end.

 

[mach4g63t]

now that really interests me. say you were to go with a stock car and raise the compression to 9.1, how much of a performance gain would there be?

I forget where I read it, but I believe it was stated that for every point of CR there is a 3% yield in horsepower (Don't quote me on that). It's nothing staggering.

So, assuming the prior, with a 300hp motor with stock internals, bolting in a set of 8.8:1 pistons (compared to 7.8:1 stockers) will net something close to 10hp. To me, that's virtually a useless upgrade unless you're planning a rebuild in the first place.

I chose 9:1 pistons for my 6-bolt over the stock 7.8:1 because I knew I was going with DSMLink as well as Methanol/Water Injection. Not only would these two allow me to increase increase my boost and timing curve, but mixed with the higher CR I would have a good bit of more power potential than I would with the same setup and stock pistons with more boost. And also, as stated in the article above from TunaTalon (he's a friggin' genius), even greater gains are to be realized when changing to a more aggressive cam when raising compression ratios.

In the end, it always comes down to money and tuning capabilities. Unless you can control your stock maps at a greater level than an AFC, raising CR will most likely only handicap your tune in the long run.

 

[Turbo Talon DL]

Using E85 insted of 90ish octane pump gas would take care of some knock issues according to all the people running the stuff. Using less timing would be great too. The stock ECU likes to advance it 40* sometimes which results in part-throttle knock.

 

[dsmjunkie97]

I have a six bolt engine that is bored .80 over 10:1 compression running 1/2 93 octane and 1/2 ethenal on 19psi and made 338whp and 344 torque with a nice fuel curve and minimal knock with a dsm link for tuning.

 

[Ryan At THM]

using e85 will also damage your valve seals. I would recommend staying away from it unless your car supports it.

 

[Turbo Talon DL]

using e85 will also damage your valve seals. I would recommend staying away from it unless your car supports it.

This is brand new to me. WTF I was actually thinking about an E85 setup too. I have 9:1 compression and that would be much better than stock ratios for the fuel. These other guys running Alch havent said anything about valve stem seals going bad. I would enjoy some links to pictures backing up these claims.

 

[90laserRSfwd]

I have a six bolt engine that is bored .80 over 10:1 compression running 1/2 93 octane and 1/2 ethenal on 19psi and made 338whp and 344 torque with a nice fuel curve and minimal knock with a dsm link for tuning.

Thought it was 0.060 over when Adam had the car? Rebuild? Are you still on the EVO III 16g?